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    <p>Quoting from the <a href="http://www.ietf.org/rfc/rfc4122.txt" rel="nofollow">UUID standard</a>:</p> <p>The structure of UUIDs is:</p> <pre> Field Data Type Octet Note # time_low unsigned 32 0-3 The low field of the bit integer timestamp time_mid unsigned 16 4-5 The middle field of the bit integer timestamp time_hi_and_version unsigned 16 6-7 The high field of the bit integer timestamp multiplexed with the version number clock_seq_hi_and_rese unsigned 8 8 The high field of the rved bit integer clock sequence multiplexed with the variant clock_seq_low unsigned 8 9 The low field of the bit integer clock sequence node unsigned 48 10-15 The spatially unique bit integer node identifier </pre> <p>In the absence of explicit application or presentation protocol specification to the contrary, a UUID is encoded as a 128-bit object, as follows:</p> <p>The fields are encoded as 16 octets, with the sizes and order of the fields defined above, and with each field <strong>encoded with the Most Significant Byte first</strong> (known as network byte order). Note that the field names, particularly for multiplexed fields, follow historical practice.</p> <pre> 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | time_low | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | time_mid | time_hi_and_version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |clk_seq_hi_res | clk_seq_low | node (0-1) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | node (2-5) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ </pre> <p>Quoting from the <a href="http://msdn.microsoft.com/en-us/library/3573497s.aspx" rel="nofollow">Guid documentation</a>:</p> <pre><code>public Guid(int a, short b, short c, byte[] d) Guid(1,2,3,new byte[]{0,1,2,3,4,5,6,7}) creates a Guid that corresponds to "00000001-0002-0003-0001-020304050607". </code></pre> <p>According to <a href="http://en.wikipedia.org/wiki/Endianness" rel="nofollow">Wikipedia's Endianness article</a> Windows stores <strong>numbers</strong> in little endian --> least significant byte first.</p> <p>The mapping 1,2,3,new byte[]{0,1,2,3,4,5,6,7} to "00000001-0002-0003-0001-020304050607", shows us that the numbers are displayed big-endian, as in the UUID standard, however, the byte array is provided in the same order as display - no need to swap the bytes.</p> <p>So Guids are displayed as:</p> <p>{time_low (4B) - time_mid (2B) - time_hi_and_version (2B) - clock_sq_hi_and_reserved(1B), clock_seq_low (1B) - node (6B)}</p> <p>In little endian this results in byte order (byte[] does not count as a number as is therefore :</p> <p>{3,2,1,0 - 5,4 - 7,6 - 8,9 - 10,11,12,13,14,15}</p> <p>which results in hexidecimal character order (each byte is 2 hexidecimal digits):</p> <p>{6,7,4,5,2,3,0,1 - 10,11,8,9 - 14,15,12,13 - 16,17,18,19 - 20,21,22,23,24,25,26,27,28,29,30,31}</p> <p>In Oracle, the <a href="http://www.techonthenet.com/oracle/functions/substr.php" rel="nofollow">substr string function</a> is 1-based, therefore the Oracle string indexes are:</p> <p>{7,8,5,6,3,4,1,2 - 11,12,9,10 - 15,16,13,14 - 17,18,19,20 - 21,22,23,24,25,26,27,28,29,30,31,32}</p> <p>Which results in the command</p> <pre><code>substr(HexData, 7, 2) || substr(HexData, 5, 2) || substr(HexData, 3, 2) || substr(HexData, 1, 2) || '-' || substr(HexData, 11, 2) || substr(HexData, 9, 2) || '-' || substr(HexData, 15, 2) || substr(HexData, 13, 2) || '-' || substr(HexData, 17, 4) || '-' || substr(HexData, 21, 12); </code></pre> <p>And transposed (after removing '-'s):</p> <p>{7,8,5,6,3,4,1,2, 11,12,9,10, 15,16,13,14, 17,18,19,20, 21,22,23,24,25,26,27,28,29,30,31,32} </p> <p>is reversed back into</p> <p>{1,2,3,4,5,6,7,8, 9,10,11,12, 13,14,15,16, 17,18,19,20, 21,22,23,24,25,26,27,28,29,30,31,32}</p> <p>using the same function (without adding '-'s) - converting BE to LE and LE to BE have the same swaps since the bytes are simply reversed and reversing reversed bytes results in non-reversed bytes.</p>
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